High tensile steel, including advanced high strength steel (AHSS), ultra high strength steel (UHSS), and high strength low alloy (HSLA) steel, is increasingly used in automotive, construction, and industrial applications due to its excellent strength to weight ratio. However, processing these materials presents significant challenges compared to conventional mild steel. This guide examines the key challenges and the equipment solutions available for successful high tensile steel coil processing.
Why High Tensile Steel Is Challenging to Process
High tensile steel grades have yield strengths ranging from 350 MPa for HSLA grades to over 1500 MPa for press hardened steels. These high strength properties create several processing difficulties.
First, the higher cutting forces required for slitting and shearing put greater stress on tooling, arbors, and machine frames. This can lead to accelerated tool wear, arbor deflection, and reduced cut quality if the equipment is not designed for the higher loads.
Second, high tensile steel has greater spring back after forming and leveling. This means that the leveler must apply more force and the roll settings must be more aggressive to achieve the desired flatness. The material also tends to retain more residual stress, which can cause shape changes after cutting.
Third, the high hardness of the material accelerates wear on slitting knives, shear blades, and leveler rolls. Tooling life is significantly shorter than for mild steel, increasing operating cost and changeover frequency.
Fourth, high tensile steel is more prone to edge cracking during slitting, especially if the knife clearance is not properly set. The sheared edge quality is critical because edge cracks can propagate during subsequent forming operations, leading to part failure.
Equipment Requirements for High Tensile Steel
Processing high tensile steel requires equipment that is specifically designed or upgraded for the higher forces and stresses involved.
Slitting Lines for High Tensile Steel
A slitting line for high tensile steel must have a rigid slitter head with large diameter arbors to minimize deflection under high cutting forces. The arbor bearings must be rated for the higher radial loads. Knife material should be high grade tool steel or carbide with appropriate surface treatment to resist wear.
The knife clearance must be precisely adjustable and set according to the material thickness and strength. For high tensile steel, the clearance is typically 8 to 15 percent of material thickness, compared to 5 to 8 percent for mild steel. Too little clearance causes excessive knife wear and heat, while too much clearance produces burr and edge cracking.
The recoiler and tensioner must provide sufficient tension to recoil the stiffer strips without telescoping. Higher tension may require more powerful drives and heavier tensioner rolls.
Levelers for High Tensile Steel
Leveling high tensile steel requires a leveler with high roll force, small diameter work rolls, and independent roll adjustment. A precision leveler with 17 to 21 work rolls is typically needed to achieve the required flatness. The leveler frame must be rigid to prevent deflection under the high forces.
For the highest strength grades, a tension leveler may be necessary. Tension leveling applies longitudinal tension in addition to roller bending, which helps to overcome the high spring back and residual stress of high tensile steel. However, tension leveling requires careful control to avoid exceeding the material tensile strength and causing fracture.
Cut to Length Lines for High Tensile Steel
Cut to length lines for high tensile steel require heavy duty shears with sufficient cutting force. Hydraulic shears are generally preferred over mechanical shears for high strength material because they provide more controlled cutting action and can handle higher forces. The shear blades should be made of high grade tool steel with appropriate hardness.
The leveler in a cut to length line for high tensile steel is particularly critical because the sheets must be flat enough for downstream stamping or laser cutting. A high precision leveler with strong roll force is essential.
Tooling and Consumables
Tooling is one of the largest operating cost factors in high tensile steel processing. Slitting knives, shear blades, and leveler rolls all wear faster when processing high strength material. Using the right tool steel grade, proper heat treatment, and surface coatings can significantly extend tool life.
Carbide tooling, while more expensive initially, can provide 2 to 5 times longer life than conventional tool steel for high tensile steel applications. Regular inspection and regrinding schedules are essential to maintain cut quality and prevent tool failure.
Process Parameter Guidelines
The following table provides general guidelines for processing common high tensile steel grades.
| Grade Type | Typical Yield Strength | Slitting Speed | Knife Clearance | Leveler Requirement |
|---|---|---|---|---|
| HSLA 350 to 450 | 350 to 450 MPa | 60 to 120 m/min | 7 to 10 percent | Standard precision leveler |
| AHSS DP 500 to 800 | 350 to 550 MPa | 40 to 100 m/min | 8 to 12 percent | High force precision leveler |
| AHSS DP 800 to 1200 | 500 to 800 MPa | 30 to 80 m/min | 10 to 15 percent | High force leveler or tension leveler |
| UHSS 1200 to 1500 | 800 to 1200 MPa | 20 to 60 m/min | 12 to 18 percent | Tension leveler recommended |
| Press hardened 1500+ | 1000+ MPa | 10 to 40 m/min | 15 to 20 percent | Specialized heavy duty leveler |
Common Quality Issues and Solutions
- Excessive burr: Caused by incorrect knife clearance or dull knives. Solve by increasing clearance for high tensile material and regrinding knives more frequently.
- Edge cracking: Caused by excessive knife clearance or material with poor edge ductility. Solve by optimizing clearance and using knife geometry designed for high strength material.
- Poor flatness: Caused by insufficient leveling force or incorrect roll settings. Solve by using a higher force leveler and optimizing roll penetration.
- Coil set memory: Caused by high residual stress. Solve by using a tension leveler or increasing leveler roll penetration.
- Tool wear: Accelerated wear is normal with high tensile steel. Solve by using higher grade tooling, carbide inserts, and more frequent regrinding.
- Shape distortion after cutting: Caused by residual stress release. Solve by improving leveling and ensuring consistent tension throughout the line.
Frequently Asked Questions
- Can I process high tensile steel on a standard slitting line? It depends on the line rating. Lines designed for mild steel may not have sufficient arbor rigidity, knife force, or tension capacity for high tensile steel. Consult the line manufacturer for material rating limits.
- What is the best knife material for high tensile steel? High grade tool steel such as D2 or A2 with proper heat treatment is common. For the highest strength grades, carbide knives or carbide inserts provide the best wear resistance.
- How does high tensile steel affect slitting speed? Higher strength material generally requires lower slitting speeds to control heat generation, maintain cut quality, and protect tooling. Speed reductions of 30 to 50 percent compared to mild steel are common.
- Do I need a special leveler for AHSS? For AHSS grades above 600 MPa yield, a high force precision leveler or tension leveler is recommended. Standard levelers may not provide enough force to achieve acceptable flatness.
- How can I reduce tooling cost when processing high tensile steel? Use the appropriate tool steel grade, maintain correct clearance, implement a regular regrinding schedule, and consider carbide tooling for high volume production. Proper lubrication also reduces wear.
Conclusion
Processing high tensile steel coils requires specialized equipment, careful parameter control, and diligent tooling management. By understanding the unique challenges posed by high strength materials, selecting equipment with adequate force and rigidity, using appropriate tooling, and optimizing process parameters, you can successfully process high tensile steel with consistent quality and reasonable operating cost. Work with equipment suppliers who have experience with high strength materials to ensure your line is properly designed and commissioned for your specific product range.